1. Field of the Invention
This invention relates to the field of electrochemistry and to photoelectrochemical cells having electrodes coated with protective and/or catalytic coatings which enhance the efficiency of the photoelectrochemical devices. More specifically, the present invention relates to an improved method of attaching such protective and/or catalytic coatings onto semiconductor electrodes including particulate semiconductor microelectrode systems.
2. Description of the Prior Art
The field of photoelectrochemistry is recognized as having the potential to enable solar energy utilization to meet many of the energy needs of the future. Through the action of light, photoelectrochemical cells can be used to generate electric power and/or to synthesize fuels and desired chemicals from abundant, renewable resources such as water, nitrogen and carbon dioxide.
Photoelectrochemical cells can be configured such that one or both electrodes are photoactive semiconductors. The electrodes are in contact with an electrolyte which may be in liquid form or may also comprise a solid polymer matrix. A junction is formed at the semiconductor-electrolyte interface in the dark as the two phases come into electronic equilibrium such that the Fermi level of the semiconductor E.sub.F equals the electrochemical potential of the solution, E.sub.redox, producing a barrier height which depends on the nature of the solution species and the specific semiconductor. On illumination of the semiconductor with light energy equal to or greater than that of the semiconductor bandgap, electrons are promoted from the valence band to the conduction band, creating electron-hole pairs at or near the interface. The electron-hole pairs are spatially separated by the semiconductor junction barrier and are injected into the electrolyte at the respective electrodes to produce electrochemical oxidation and reduction reactions.
A major impediment to the exploitation of photoelectrochemical cells in solar energy conversion and storage is the susceptibility of small bandgap semiconductor materials to photoanodic and photocathodic degradation. The photoinstability is particularly severe for n-type semiconductors where the photogenerated holes which reach the interface, can oxidize the semiconductor itself. In fact, all known semiconducting materials are predicted to exhibit thermodynamic instability toward anodic photodegradation. Whether or not an electrode is photostable then depends on the competitive rates of the thermodynamically possible reactions, namely, the semiconductor decomposition reaction and the electrolyte reactions.
Examples of photoanodic decomposition reactions are compiled in Table I.
TABLE 1 ______________________________________ Examples of Photoanodic Decomposition Reactions of Various Semiconductor Electrodes Decomposition Semiconductor Photoanodic Process ______________________________________ Si Si + 4h.sup.+ + 2H.sub.2 O .fwdarw. SiO.sub.2 + 4H.sup.+ GaAs GaAs + 6h.sup.+ + 5H.sub.2 O .fwdarw. Ga(OH).sub.3 + HAsO.sub.2 + 6H.sup.+ GaP GaP + 6h.sup.+ + 6H.sub.2 O .fwdarw. Ga(OH).sub.3 + H.sub.3 PO.sub.3 + 6H.sup.+ CdS CdS + 2h.sup.+ .fwdarw. Cd.sup.2+ + S CdSe CdSe + 2h.sup.+ .fwdarw. Cd.sup.2+ + Se MoS.sub.2 MoS.sub.2 + 18h.sup.+ + 12H.sub.2 O .fwdarw. MoO.sub.3.sup.2- + 2SO.sub.4.sup.2- + 24H.sup.+ WO.sub.3 WO.sub.3 + 2h.sup.+ + 2H.sub.2 O .fwdarw. WO.sub.4.sup.2- + 1/2O.sub.2 + 4H.sup.+ ______________________________________
The range of approaches for suppression of the photocorrosion problem in cells for chemical production is more severe than that for electricity-generating cells. This is particularly true if the electrolyte contains water. Table II illustrates some examples of fuel-producing reactions in aqueous electrolytes.
TABLE 2 __________________________________________________________________________ Some endergonic fuel generation reactions starting with N.sub.2, CO.sub.2 and H.sub.2 O H.degree. G.degree. Reaction (kJ mol.sup.-1).sup.a (kJ mol.sup.-1).sup.a __________________________________________________________________________ ##STR1## 286 237 ##STR2## 270 286 ##STR3## 563 522 ##STR4## 727 703 ##STR5## 890 818 ##STR6## 765 678 ##STR7## 467 480 __________________________________________________________________________ 1 V = 23.06 K cal/mol - 96.485 kJ/mol 1 J = 0.23901 cal
Water is a particularly attractive source of hydrogen for the reduction of materials such as N.sub.2 and CO.sub.2 as well as for the direct generation of H.sub.2. Water can only be used, however, if the semiconductor electrodes are stable in its presence. In the illustrations, the production of energy rich materials (e.g. H.sub.2, CH.sub.3 OH, CH.sub.2 O.sub.3, CH.sub.2 O.sub.2 and NH.sub.3) is associated with O.sub.2 evolution. A major problem in photoelectrochemistry is that the oxidation of water at the photoanode of nonoxide n-type materials is thermodynamically and kinetically disfavored over the reaction of the valence band holes with the semiconductor lattice. In fact, all known nonoxide and many oxide n-type photoanodes are susceptible to photodegradation in aqueous electrolytes.
Approaches have been used to control the photoinstability of the semiconductor-electrolyte interface by coating the semiconductor surface. For example, to stabilize semiconductor surfaces from photodecomposition, noncorroding layers of metals or relatively stable semiconductor films have been deposited onto the electrode surface. It has been reported that continuous metal films which block solvent penetration can protect n-type GaP electrodes from photocorrosion. However, if the films are too thick for the photogenerated holes to penetrate without being scattered, they assume the Fermi energy of the metal. Then the system is equivalent to a metal electrolysis electrode in series with a metal-semiconductor Schottky barrier. In such a system, the processes at the metal-semiconductor junction control the photovoltage and not the electrolytic reactions. In general, a bias is required to drive the water oxidation. In other cases, the metal can form an ohmic contact that may lead to the loss of the photoactivity of the semiconductor. In discontinuous metal coatings, the electrolyte contacts the semiconductor, a situation which can lead to photocorrosion. For example, discontinuous gole films do not seem to protect n-type GaP from photocorrosion.
Corrosion-resistant wide-bandgap oxide semiconductor (TiO.sub.2 and titanates mostly) coatings over narrow bandgap n-type semiconductors such as GaAs, Si, CdS, GaP, and InP have been shown to impart protection from photo-decomposition. One of two problems is currently associated with the use of optically transparent, wide-bandgap semiconducting oxide coatings: either a thick film blocks charge transmission or a thin film still allows photocorrosion.
Wrighton et al. (1978) have shown that chemical bonding of an electroactive group to an n-type semiconductor surface can reduce oxidative photocorrosion of the electrode during electrical power generation. However, the electroactive group consisted of ferrocene molecules which are not polymeric. When a polymeric material containing a catalyst was covalently attached to the electrode surface, the polymer was not electrically conductive and the electrode was p-type [Dominey et al. J. Am. Chem. Soc. 104, 467 (1982)]. This distinction is important because with p-type electrodes, photodegradation by reductive processes is not a major problem in photoelectrochemical solar energy utilization. In the case of n-type and p-type semiconductors coated directly with thin catalytically active metal films for gaseous fuel production, and generally poor adherence of the metal to the semiconductor surface is a major impediment.
Charge conduction is generally much higher in electrically conductive polymers than in typical electroactive polymers. Accordingly, work on charge-conductive polymers in the field of photoelectrochemistry has been directed toward stabilization of electrodes against photodegradation in electricity generating cells. Charge-conductive polymers are known to protect certain semiconductor surfaces from photodecomposition by transmitting photogenerated holes in the semiconductor to oxidizable species in the electrolyte at a rate much higher than the thermodynamically favored rate of decomposition of the electrode. For example, R. Noufi, A. J. Frank, A. J. Nozik [J. Am. Chem. Soc., 103, 1849 (1981)] demonstrated that coating n-type silicon semiconductor photoelectrodes with a charge-conductive polymer, such as polypyrrole, enhances stability against surface oxidation in electricity generating cells. As also reported by R. Noufi, D. Tench and, L. F. Warren, [J. Electrochem. Soc. 127, 2310 (1980)], n-type GaAs has also been coated with polypyrrole to reduce photodecomposition in electricity-producing cells, although the polymer exhibited poor adhesion in aqueous electrolyte.
Preferred methods to deposit the polymer on the electrode surface include in situ synthesis or polymerization of the coating by submersing the electrodes in monomer solution and initiating a current flow through the circuit. Where the electrodes are photoelectrodes light may be required to induce such a current.
The nature and the strength of the interaction between the semiconductor and the surface coating effect the adhesion and the efficiency of charge transfer at the interface and thus the stability of the semiconductor. Polypyrrole films exhibit substantially stronger adhesion to polycrystalline Si than to single-crystal Si because of various physical and chemical factors associated with the surface of the substrates. [A. J. Frank in "Molecular Crystals and Liquid Crystals" (A. J. Epstein and E. M. Conwell, eds.), Vol. 83, Gordon & Breach Science Publishers, New York, 1982. p. 1373] Platinum [T. Skotheim, I. Lundstrom and J. Prejza, J. Electrochem. Soc. 128, 1625 (1981)] and gold [F. F.-R. Fan, B. L. Wheeler, A. J. Bard and R. Noufi, J Electrochem Soc. 128, 2042 (1981)] metallization of single-crystal Si prior to anodic polymerization of pyrrole increases the adhesion of the film during power generation. However, during the synthesis of gaseous fuels, such surface deposits of noble metals underlying polymer films can serve as catalytic sites for gas generation which can physically disrupt the polymer-substrate interaction and thus lead to the detachment of the film. Moreover, the use of a metal underlayer to improve the adhesion of the polymer to the substrate has been limited to Si where the adhesion of the metal to the Si is favorable; the general application of the method to other types of semiconductors may not be possible. Another possible limitation of the method is that the high density of electronic states of metal films can adversely affect the interface energetics of the semiconductor and the electrolyte by leading to Fermi-level pinning and thus deleteriously affect the fuel generating reaction.
Despite the promising use of polypyrrole on n-type silicon to suppress photodecomposition, heretofore, the ability to adequately adhere conductive polymers alone or as used in conjunction with catalysts has been uncertain.